The present invention relates generally to the field of orthopaedics, and more particularly, to an implant for use in arthroplasty.
The invention relates to implantable articles and methods for manufacturing such articles. More particularly, the invention relates to bone prostheses and processes for manufacturing the same.
There are known to exist many designs for and methods for manufacturing implanted articles, such as bone prostheses. Such bone prostheses include components of artificial joints, such as elbows, hips, knees, and shoulders. An important consideration in the design and manufacture of virtually any implantable bone prosthesis is that the prosthesis has adequate fixation when implanted within the body.
Early designs of implantable articles relied on the use of cements, such as polymethylmethacrylate to anchor the implant. The use of such cements can have some advantages, such as providing a fixation that does not develop free play or does not lead to the erosion of the joining bone faces post-operatively. However, the current trend is to use the cements to a lesser extent because of their tendency to lose adhesive properties over time and the possibility that the cement contributes to wear debris within a joint. Implantable articles, which are not implanted with the use of cements, are implanted by providing a cavity for the implant that matches the geometry of the implant and then press fitting the implantable article into the cavity.
Whether utilizing cement or the press-fitting technique for the implantable articles, a problem has been observed which relates to the proper distribution of stresses within the prosthesis and throughout the surrounding bone. This problem can best be described with references to Wolff""s Law. This Law is generally based on the human anatomy of principles of atrophy. According to the principle of atrophy, when a human tissue is under utilized, it will atrophy or deteriorate. Conversely, as human tissue is utilized, it will grow and strengthen. According to Wolff""s Law, this phenomenon can describe the conditions on human bone, particularly when that bone is used in conjunction with an implantable prosthesis.
According to Wolff""s Law, atrophy is defined as the thinning of the cortex with retention of normal cortical texture. According to Wolff""s Law, hypertrophy will occur at the area of highest stress surrounding an implant. The thickening of the cortex is a very desirable event in the post-operative patient. For many implants within a long bone, the location of hypertrophy is often at that distal end of the implant. This is caused by the artificially raised stresses at the point of sudden transition from the flexible distal femur to the artificially stiffened proximal femur. This is true both for press fit and cemented stems. This phenomenon of hypertrophy does result in excellent adhesion in the diaphysis but results in a less than desirable condition between the implant and the long bone in the metaphesis.
If too little stress is applied to the bone, resorption can occur leading to atrophy of the affected area. Too much stress may also lead to resorption in atrophy, or may result in an undesirable atrophy of the affected area. Accordingly, there exists a need for an improved joint prosthesis, that addresses the needs and problems of prior joint designs as it relates to the distribution of stress.
The phenomenon of atrophy resulting from insufficient loading at certain portions of the implant and bone interface has been referred to as stress shielding.
Stress shielding has been addressed by putting a porous coating only on the proximal portion of the joint prosthesis and using a highly refined surface on the distal surface of the prosthesis. The porous coating is utilized to encourage the growth of hard tissue around the implant. The bone attachment usually occurs and growth is promoted when the surface of the implantable bone prosthesis is regular or textured. The interactions of newly formed hard tissue in or around the texture surface of the implantable bone prosthesis has been found to provide a good fixation for the prosthesis within the bone.
The designs where the porous coating is placed only on the proximal portion of the bone of the joint prosthesis attempt to duplicate the natural transmission of the load in the long bone. Such design may be associated with leg pain.
Attempts to reduce stress shielding include various attempts to make the stem more flexible. These efforts have included the clothespin design or central opening in the prosthesis. Another attempt is the use of altering the cross section of the stem of the prosthesis along its length. Another method of making the stem more flexible is the use of alternate materials.
These efforts, which are aimed at making the stem more flexible, are expensive and in each case allow the distal portion of the prosthesis to rub against the inside of the bone canal as the bone flexes. This may be painful to the patient and may induce a natural bone growth near the distal end of the prosthesis as the prosthesis rubs against the inside of the bone canal.
These attempts at making the stem more flexible by making the stem smaller or of a material with less strength may increase the probability that the stem will fracture in the long bone. Further, the use of the more flexible material may require that their size be larger and they may require that a larger amount of bone must be reamed from the bone canal.
The natural anatomy of the long bone is arcuate and curved in the central portion of the long bone. For use in revision surgery and when a fracture has occurred in the long bone, prostheses have been developed which are fitted into the more distal curved portion of the long bone. Such stems have been designed with curved distal portions. It can be difficult to prepare the long bone canal to accept a curved stem and accurate insertion can therefore be challenging. Devices with these features are expensive to produce and a large number of sizes and shapes are required to fit the individual anatomy including the need to obtain the proper ante-version in the patient.
Efforts have been made to reduce the instance of point stress associated with the distal portion of the prosthesis stems. As mentioned earlier, the point stress may lead to leg pain and also such high point stress is located at the distal portion of the prosthesis stem. The distal point stress may lead to fracture resulting from trauma or sub optimal bone quality. Bullet-shaped tips have met with some success but still cause some stress concentration at the distal portion of the implant.
While attempts have been made to improve the physiological loading of the bone to reduce stress shielding, the prior art efforts have met with limited success.
Accordingly, a new prosthesis is needed which improves the physiological loading of the bone and thereby reduces stress shielding type pain, as well as the probability of a stem fracture of the long bone.
The present invention is an element in the form of a linear bearing that is placed in the canal of the long bone. It is designed to mate with the stem of a prosthetic joint or an intermedullary rod. Typically, stems or rods used with this device would be smaller in diameter than existing orthopedic implants and would not contact the interior cortex of a long bone. Instead, the present invention would allow the stem or rod to guide or slide up and down in a central bushing.
One or more of the linear bearings may be used in conjunction with a single stem or intermedullary rod. The device allows a long bone, for example, a femur, to bend under load without shielding the bone from the normal stress patterns nearly as much as conventional designs of orthopedic joint implants do. The present invention places compressive forces almost exclusively in the proximal portion of the prosthetic joint. The linear bearing may include an articulating feature within the canal of the long bone.
The outer surface of the linear bearing of the present invention may be porous coated. The porous coating is designed to promote bone in-growth. The inner diameter of the linear bearing of the present invention is a bearing surface that captures the stem of a prosthetic joint member and holds it in axial alignment, but allows the stem to move up and down as psychological loads are applied to the long bone causing the long bone to bend. Accordingly, the stem of the prosthesis does not fill the bone canal and the stem may be made more flexible than current stem designs.
The linear bearing of the present invention does not require the canal of the long bone to be reamed down to the cortical bone over the length of the stem. Instead, an area in the bone canal may be prepared to receive the linear bearing with a hone that may be inserted into the canal at a depth where the linear bearing is to be placed. Centrifugal force may then be used to expand the hone as power is applied. The hone could be designed with shoes to prevent too much penetration of cortical bone. Alternately, a boring bar could be used for the same purpose. The boring bar can be inserted into a bore to make a larger diameter bore down lower.
The linear bearing of the present invention may be in the form of a simple -C- that is designed to be hammered down into position (or compressed and released into position) with the inner surface somewhat convex to accommodate the small deviation in placement where the center line of the linear bearing is not exactly parallel to the axis of the long bone canal.
According to one embodiment of the present invention, a joint prosthesis for cooperation with a long bone and a second bone for use in arthroplasty is provided. A cavity is formed in the long bone and is defined by an inner wall of the long bone. The prosthesis includes a stem component including a distal portion of the stem component for placement at least partially within the cavity of the long bone and for securing it to the long bone. The stem defines a longitudinal axis of the stem. The prosthesis also includes a second component securable to the second bone and for cooperation with the stem component. The prosthesis further includes a bearing for placement in the cavity between the stem and the long bone so that the distal portion of the stem component is spaced from the inner wall of the long bone.
According to another embodiment of the present invention, a hip joint prosthesis for cooperation with a femur and an acetabulum for use in arthroplasty is provided. A cavity is formed in the femur and is defined by an inner wall of the femur. The prosthesis includes a femoral component having a portion of the femoral component for placement at least partially within the cavity of the femur. The femoral component defines a longitudinal axis of the femoral component. The prosthesis also includes a cup for attachment to the acetabulum and for cooperation with said femoral component. The prosthesis further includes a bearing for placement in the cavity between the femoral component and the femur so that the distal portion of the femoral component is spaced from the inner wall of the femur.
According to yet another embodiment of the present inventions stem component for use in a joint prosthesis for cooperation with a long bone for use in arthroplasty is provided. A cavity is formed in the long bone. The stem component has portions of the stem component for placement at least partially within the cavity of the stem component. The stem component defines a longitudinal axis of the stem component. The distal portion of the stem component is spaced from the long bone.
According to another embodiment of the present invention, a bearing for use in a joint prosthesis for use in arthroplasty is provided. The bearing is adapted for cooperation with a stem component placed at least partially in a cavity formed in a long bone and defined by an inner wall of the long bone. The bearing is adapted for placement in the cavity between the stem component and the long bone so that the distal portion of the stem component is spaced from the inner wall of the long bone.
According to a further embodiment of the present invention, a method for performing joint arthroplasty including the steps of preparing a cavity in the medullary canal of a long bone, providing a bearing for implantation within the cavity, installing the bearing in the cavity, providing a stem for implantation at least partially within the cavity, and installing the stem to the bearing so that the distal portion of the stem is spaced from the long bone is provided.
The technical advantages of the present invention include the physiological loading of the bone in order that stress shielding may be reduced which may lead to retention of natural bone and the potential increase in thickness of cortical bone.
For example, according to one aspect of the present invention, a stem or rod is utilized with a linear bearing that is positioned within the medullary canal of a long bone such that the stem or rod may slide up and down in a central bushing. The device allows a long bone to bend under load without shielding the bone from normal stress patterns such as that of a press fitted conventional stem.
Thus, the present invention provides for a prosthetic joint component, which improves the physiological loading of the bone, thus reducing stress shielding.
Another technical advantage of the present invention is the reduced leg pain. For example, according to one aspect of the present invention, the stem or rod used in conjunction with the linear bearing of the present invention is smaller in diameter than the medullary canal, and thus, the stem or rod is not in contact at its distal point with the bone. Without the linear bearing the contact of the stem to the bone distally may cause a portion of a prosthesis stem to rub against the inside of a bone canal as the bone flexes, causing pain. Thus, the present invention provides for reduced leg pain caused by the rubbing of a stem against the inside of the bone canal.
Another technical advantage of the present invention is the reduced probability that a stem will fracture in the long bone as a result of trauma. For example, accordingly to one aspect of the present invention, the stem or rod used in conjunction with the linear bearing of the present invention is smaller in diameter distally than the canal of the long bone. The use of a stem or rod smaller than the canal reduces the force transmitted to the long bone as a result of trauma and thus reduces the probability that a stem will fracture as a result of trauma. Thus, the present invention provides for a reduction in the probability that a stem will fracture the long bone as a result of trauma.
Additionally, the present invention reduces the amount of bone that must be reamed from the bone canal. For example, according to one aspect of the present invention, a hone can be inserted into the canal at a depth where the linear bearings are to be placed and only that portion of the long bone needs to have bone removed so that the linear bearings may be placed at that location. Thus, the present invention reduces the amount of bone that must be removed from the bone canal.
In addition, the present invention includes the technical advantage of allowing straight stems smaller than the canal diameter to be inserted past curved portions of the long bone and still remain effective. For example, according to one aspect of the present invention, the stems or rods used with the linear bearing are smaller in diameter than current designs and are allowed to be smaller than the bone canal. Thus, straight stems may be inserted past curved portions of the long bone.
Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, descriptions and claims.